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Polymorphism Example 2 - Classes Circle, Cylinder & Sphere
- Make area() a virtual function in circle.h
- In the driver program create a pointer to a base class object
and have it "point" to objects of each class.
In the definition of the Circle class:
#ifndef CIRCLE_H
#define CIRCLE_H
// Circle class definition - file circle.h
class Circle
{
friend double distance( Circle&
c ); // friend function
friend
ostream& operator<<(ostream& out, Circle& cc );
public:
Circle();
// default constructor
Circle(int x, int y, double r); // constructor
void showdata(ostream& out) const;
double get_radius() const;
// accessor
void set_radius( double r );
// mutator
void setCircle(int x, int y, double r);
virtual
double area() const;
double circumference() const;
bool equal( Circle& c2 ) const;
// operator functions
bool operator==(Circle& c2) const;
Circle operator+(Circle& circle2 ) const;
protected:
int xcenter;
int ycenter;
double radius;
static double PI; // static data member
};
#endif // CIRCLE_H
Create the Sphere class:
% cat sphere.h
#ifndef SPHERE_H
#define SPHERE_H
class Sphere: public Circle
{
public:
Sphere();
Sphere(int x, int y, double r);
double area() const;
};
#endif
% cat sphere.cpp
// Sphere class implementation - file sphere.cpp
#include<iostream>
#include<iomanip>
using namespace std;
#include "circle.h" // header file
#include "sphere.h"
// constructors
Sphere::Sphere() : Circle() {} // call base class constructor
Sphere::Sphere(int x, int y, double r)
: Circle( x, y, r) {} // call base class constructor
double Sphere::area() const
{
// surface area of sphere
return 4.0*PI*radius*radius;
}
Driver program:
% cat poly_ex.cpp
#include<iostream>
using namespace std;
#include "circle.h"
#include "cylinder.h"
#include "sphere.h"
int main()
{
cout << "Some examples using static bindings, e.g.,
circ1.area() " << endl;
Circle A(1, 1, 1.0);
cout << "Circle area " << A.area() << endl;
Cylinder B(1, 1, 1.0, 2.0);
cout << "Cylinder area "<< B.area() <<
endl;
Sphere C(2, 2, 1.0);
cout << "Sphere area " << C.area() << endl;
cout << "Some attempts at polymorphism, e.g., base_ptr->area()
" << endl;
// first create a "base class pointer"
Circle *base_ptr;
// point to a Cylinder object
base_ptr = &B;
// invoke the area function
cout << "Cylinder area = " << base_ptr->area()
<< endl;
// point to a Sphere object
base_ptr = &C;
// invoke the area function
cout << "Sphere area = " << base_ptr->area()
<< endl;
return 0;
}
Compile and execute:
% g++ poly_ex.cpp circle.cpp cylinder.cpp sphere.cpp
% a.out
Some examples using static bindings, e.g., circ1.area()
Circle area 3.14159
Cylinder area 18.8495
Sphere area 12.5664
Some attempts at polymorphism, e.g., base_ptr->area()
Cylinder area = 18.8495
Sphere area = 12.5664
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